The present invention relates to a process for purifying or separating gases or gas mixtures using a monolithic adsorbent having a cellular structure with an open porosity in the form of a solid foam permeable to the gas molecules.
At the present time, industrial adsorbents are used in the form of a granular bed. In this configuration, the bed of adsorbent acts as a loose stack of particles of any shape: beads, extrudates or more complex shapes.
This arrangement may be defined as consisting of locally an essentially convex solid phase coexisting with an essentially concave gas phase occupying the space left free by the solid.
One development approach consists in making what are known as honeycomb monolith adsorbents, essentially consisting of a solid structure and a gas phase placed essentially linearly in what are called “channels”.
A similar approach uses lamellar structures in the form of “multilayers”, with the two phases lying in sheets with a two-dimensional gas flow.
However, the basic problem of solid-phase adsorption processes, to which each of the three known arrangements does provide a solution, consists in accomplishing good solid/gas material exchange while maintaining a low head loss.
Thus, a granular bed ensures good material transfer in the gas phase but at the cost of a high head loss.
Moreover, it is difficult to greatly improve the situation as the primary cause thereof is the large size of the solid phase relative to the gas phase.
Furthermore, the solution consisting in reducing the size of the particles quickly reaches its limit due to the head loss increasing excessively. In fact, the convex, or even spherical, structure of the solid phase means that there is a low surface-to-volume ratio intrinsically unfavourable to diffusion in the solid and a high surface-to-volume ratio in the gas phase, whereas in fact it is the opposite that ought to be achieved.
From its standpoint, a honeycomb or multilayer structure is essentially linear and the two phases are therefore in an equivalent arrangement. The flow of the gas is much more uniform than in the case of a granular bed, which results in a low head loss. However, it is found in practice that the transport of matter in the gas phase is not truly satisfactory because of this uniform, that is to say laminar, flow unless channels having a size of around 0.1 or 0.2 mm are used, which poses manufacturing problems.
Starting from this point, the problem that arises is then to be able to have a gas separation or purification process using an adsorbent possessing optimized mass exchange between the gas phase and the solid phase, while maintaining the head losses at a low level when using this adsorbent to separate or purify a gas or gas mixture, in particular a PSA or VSA process, to produce, for example, oxygen or hydrogen or a TSA process used, for example, to remove carbon dioxide, nitrogen oxides and/or hydrocarbons, or water, from gases such as air or H2/CO mixtures.